What a sub-dam is
A sub-dam, fuku-entei in Japanese, is a low secondary dam built a short distance downstream of a main erosion-control dam. Water pouring over the crest of the main dam falls with great force and, left alone, would dig deep into the bed immediately below, undermining the foot of the dam. The sub-dam holds the water level up on the downstream side and absorbs the energy of the fall, preventing that scour.
It is designed as part of a single installation with the main dam. The bed between the two is covered with stone paving or concrete, forming an apron, and the banks are protected by revetments. The record for Dam No. 4 on the Mannai River describes a sub-dam in front of the main dam, joined to it by a stone-paved apron and a stone revetment, which shows the arrangement clearly.
Water falling through the spillway notch cut into the centre of the main dam lands on the apron and then passes over the sub-dam downstream. Only with main dam, notch, apron, sub-dam and revetments together is the system for carrying water safely downstream complete.
Reading the records and the site
Records for registered erosion-control dams often list the sub-dam as an attached feature in the structure line, noting revetments on both banks downstream and a sub-dam. Dams Nos. 1, 6 and 10 on the Mannai River are recorded in this way. Commentaries sometimes note condition; for Dam No. 1 the sub-dam and revetments are said to survive in good order.
Small dams have them too. The first Tanukisawa stone dam on the Shohoji River in Tottori is only 2.4 metres high, yet its record praises the thoroughness of its sub-dam and revetments. Where two dams stand side by side, as with Dams Nos. 7 and 8 on the Mannai River, one may be given an apron and revetments and the other a sub-dam, so the combination varies from dam to dam.
On site, look downstream of the main dam for a low step crossing the river a little way off. That is the sub-dam, and if the bed between it and the main dam is paved flat with stone, that paving is the apron.
References
- 万内川四号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003214
- 万内川一号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003212
- 万内川七号及び八号堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00003216
- 正法寺川狸沢第一号石堰堤｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00004865
Last verified: 2026.09.26